Communications signals, for example, mobile radio telephone signals, are often disturbed by noise. This noise may be caused by a variety of factors, such as a poor connection or ambient noise around the microphone. For example, ambient noise in an automobile may be picked up by the telephone and transmitted as background noise.
There are several methods known in the art for eliminating this noise. For example, a high-pass filter may be used to remove low frequency noise. However, when the mobile telephone receives a signal from cellular stations, the generated noise is sometimes within the same high frequency range as the speech signal. Thus, the noise is not eliminated by the high-pass filter and cannot be eliminated without removing the speech as well.
Efforts have been made to eliminate automobile background noise. One method places a second microphone (e.g., a second "telephone mouthpiece") at a location remote from the user so that this microphone picks up only ambient noise. This second microphone's signal is inverted and summed with the signal from the first microphone to cancel or eliminate background noise from the entire signal before transmitting the speech signal. However, this method results in only a slight improvement in the signal-to-noise ratio because it is often difficult to place the microphone in a suitable place where it will pick up the correct amount and type of noise without also picking up and cancelling part of the voice signal.